Method for controlling the elevation of a retaining device of a ground milling machine, and ground milling machine

ABSTRACT

A method for controlling the elevation of a retaining device of a ground milling machine as well as to a ground milling machine. Essential for the invention is that the retaining device is held in an initial position at a distance from the underlying ground and is reset from a yield position.

FIELD

The invention relates to a method for controlling the elevation of a retaining device of a ground milling machine as well as to a ground milling machine.

BACKGROUND

Generic ground milling machines are known in the prior art, for example from EP 3168367 A1 and DE 19726122 A1. Such ground milling machines are used for milling road surfaces, in strip mining operations, in stabilization and recycling applications in street and roadway construction as well as in road surface restoration. Depending on the field of application, such ground milling machines are also referred to as cold milling machines, surface miners, stabilizers or recyclers.

Ground milling machines of this type comprise a machine frame supported by traveling devices with front and rear traveling units. The traveling devices can be connected to the machine frame at least in part by means of height-adjustable lifting columns, so that the machine frame can be lowered and raised in relation to the ground underneath. Also arranged on the machine frame is an operating platform from which the ground milling machine is operated. The ground milling machine further comprises a milling device connected to the machine frame with a milling drum arranged inside a milling drum housing, said milling drum being rotatable about a rotation axis extending transversely to the working direction. The rotation axis usually extends horizontally and transversely to the main working direction. This is usually the forward direction of the ground milling machine. Also provided is a drive unit, usually a diesel combustion engine, which generates the drive power required for the milling and traveling operations. The ground milling machine is usually designed as a self-propelled ground milling machine.

Possible variations exist, inter alia, with respect to the arrangement of the milling device. In so-called rear-rotor milling machines, the milling device can be arranged in the rear area, in particular at the level of the two rear traveling devices. Such a ground milling machine is described, for example, in DE 19726122 A1. Also known are so-called center-rotor milling machines, in which the milling device is arranged between front and rear traveling devices at a distance from the same in the main working direction. Such a ground milling machine is disclosed in greater detail, for example, in EP 3168367 A1.

Generic ground milling machines further frequently comprise a transport device with which the milled material obtained during the milling operation can be loaded, for example, onto a transport vehicle. This can be done in the main working direction to the front as well as to the rear or laterally.

An essential element of generic ground milling machines is the milling device. This comprises a milling drum arranged inside the milling drum housing. The milling drum housing usually comprises delimiting walls to the sides in the form of so-called side plates that are height-adjustable in relation the ground, a front delimiting wall and a rear stripping plate, in order to prevent milled material from flying around and to facilitate, as far as possible, a complete removal of the milled material. In generic ground milling machines, the provision of a retaining device that is arranged in front of the milling drum in the milling direction and that is height-adjustable via an actuator is also known. Said retaining device can have, for example, overall a structure like a sliding block, which closes off the front delimiting wall comprising the milling drum housing to the front in certain circumstances. The object of the retaining device is primarily in particular to prevent ground material, in particular clumps of the same, from being raised in front of the milling drum in the milling direction in order to facilitate a reliable milling operation as well as in particular a reliable breaking up of the milled material inside the milling drum housing. The retaining device is height-adjustable in the vertical direction. With regard to the control of its elevation, the retaining device conventionally skims over the underlying ground in a kind of floating position during the milling operation and is raised temporarily if necessary, as described, e.g., in DE 19814053 A1. It is also known to press the retaining device against the ground with increased force if necessary, as described in EP 3168367 A1. The retaining device can also act as a mount for a conveyor belt.

The known systems for controlling a retaining device described above are, however, disadvantageous inasmuch as the retaining device, which usually scrapes over the underlying ground in front of the milling drum throughout the milling operation, wears out quickly, in particular the runners frequently provided on the same, and additionally reduces the forward propulsion of the ground milling machine.

SUMMARY

The object of the invention is thus to indicate a way to reduce wear and tear on the retaining device and to further optimize the milling process. At the same time, in particular the formation of ground clumps during the milling operation should be avoided or at least reduced.

The object is achieved with the method for controlling the elevation of a retaining device of a ground milling machine and with a ground milling machine according to the independent claims. Preferred embodiments are indicated in the dependent claims.

The basic idea of the invention is, instead of keeping the retaining device in a standard position in which it is in contact with the ground or even pressed down against the ground during the milling operation, rather to guide it over the underlying ground in a slightly raised position throughout the milling process. This has the advantage that any wear on the retaining device is reduced considerably and at the same time the forward propulsion of the ground milling machine is no longer reduced by a retaining device permanently dragging on the underlying ground.

According to the inventive method, the retaining device is thus held in an initial position at a defined holding distance above the underlying ground in a step A) before the initiation of a milling operation, so that the retaining device is not in contact with the underlying ground. An essential aspect of this step is thus that the underside of the retaining device facing the underlying ground no longer rests on the ground surface but is rather raised and held at the defined holding distance, which designates a vertical distance from the ground contact side of the retaining device to the underlying ground. The distance indicated here relates to the unprocessed ground substrate. If the ground milling machine is now moved in the working direction in a step B), during which the milling process is initiated, the retaining device, which is raised off the underlying ground in the initial position, no longer scrapes over the underlying ground. The holding distance here is selected so that the gap between the underlying ground and the bottom edge of the retaining device, which is formed when raising the retaining device, is relatively small, so that no milled material, or at least no significant amount of milled material, is conveyed to the front and out of the milling drum housing. Such a slightly raised positioning of the retaining device, which is the default setting, is advantageous in that it leads to considerably less wear and, moreover, does not constitute a resistance to the forward propulsion of the ground milling machine in the main working direction.

The slightly raised positioning of the retaining device in the initial position described above is generally maintained throughout the milling process. However, if, in step C), a force is exerted on the retaining device in a vertical upward direction, which can occur, for example, when ground material, especially in the form of clumps, is pushed from below against the retaining device by the milling drum, which generally rotates inversely to the main working direction, so that this material presses against the underside of the retaining device from below with a vertical upward counterforce, the retaining device is initially kept in the initial position with a retaining force up to a defined maximum retaining force in accordance with the invention. In other words, a retaining force, which counteracts the upward counterforce exerted by the ground material, is actively applied to the retaining device. As long as the maximum retaining force is not exceeded, the retaining device thus remains in its initial position. However, if the vertical upward counterforce exceeds the maximum retaining force, a release of the retaining device for vertical upward displacement occurs in step D), and the retaining device is pushed upwards by the counterforce. In other words, the retaining device thus yields in the upward direction in this case and is further raised from its initial position, i.e. is pushed by the material into a position in which it is raised further. This scenario can occur, for example, when the milling operation temporarily produces exceptionally large clumps of material or the like. However, in accordance with step E), the retaining device is returned towards the initial position when the counterforce falls back below the maximum retaining force, the retaining device being returned no farther than the initial position, i.e. not to a lowered position at the level of the unprocessed ground surface. Thus, if the counterforce exerted on the retaining device by the ground material decreases until it falls below the maximum retaining force, the retaining device is pressed back downwards, however no farther than to its initial position. This pressing in the downward direction as well as the exertion of the retaining force can already be effected, for example, by the mere weight of the retaining device. Preferably, however, there is an active application of force on the retaining device in the vertical downward direction, for example with the help of the hydraulic height-adjustment system of the retaining device.

Overall, the method according to the invention thus facilitates a relatively low-wear operation of the retaining device and at the same time an increased forward propulsion efficiency of the ground milling machine.

Advantageous embodiments of the method according to the invention relate inter alia to the positioning of the retaining device in the initial position prior to the beginning of the milling process. Various alternatives can additionally or alternatively be used here. For example, step A) is preferably preceded by a setting down of the retaining device on the underlying ground and a subsequent raising of the retaining device until the initial position at the defined holding distance of the retaining device from the underlying ground is reached. The setting down of the retaining device can be detected very reliably and with simple means, for example by means of pressure sensors in the hydraulic height-adjustment system of the retaining device. The position in which the retaining device is resting on the underlying ground thus corresponds to the zero position. From this position, it can now be raised to the initial position in a defined manner. This can occur, for example, as a function of an actuation distance and/or time. Additionally, or alternatively, it is also possible to measure an operating variable associated with the height adjustment of the retaining device and to set the lifting height of the retaining device until a target value of the operating variable associated with the height adjustment of the retaining device is reached, said target value corresponding to a reaching of the defined holding distance of the initial position. Such an operating variable can be, for example, an actuation distance of a lifting cylinder of the retaining device or the like. To this end, it is possible to use either the associated operating variable directly or an actual raised position of the retaining device derived from the associated operating variable. Additionally, or alternatively, it is also possible to measure a distance variable that changes with the height adjustment of the retaining device in relation to the underlying ground, and to raise or lower the retaining device until a distance variable corresponding to the defined holding distance of the initial position is reached. This can occur, for example, by means of a distance sensor, for example a laser- or ultrasound-based distance sensor, which is mounted on the retaining device or on a part that is adjustable together with the retaining device, and which ascertains, directly or indirectly, an actual distance of the retaining device from the underlying ground in the vertical direction. Additionally or alternatively, it is further possible that an actual raised state of the retaining device is derived with the aid of displacement sensors associated with lifting columns for traveling devices and/or with the aid of a displacement sensor associated with a lifting device of the retaining device and/or with the aid of a position sensor of the ground milling machine, and the retaining device is raised or lowered until a target raised state of the retaining device corresponding to the defined holding distance of the initial position is reached and/or until a target displacement value of the displacement sensor associated with the lifting device of the retaining device is reached, which corresponds to the defined holding distance of the initial position, in particular while taking into account measurement data of the position sensor of the ground milling machine.

In order to keep the retaining device in the initial position at a defined holding distance above the underlying ground, a pressure force acting in the vertical downward direction is preferably actively applied in step C) in addition to the weight of the retaining device. This means that the maximum retaining force exceeds the force exerted by the mere weight of the retaining device in the vertical downward direction, in particular by a factor greater than 1.3. In this manner, the load that can be applied to the retaining device until the retaining device is released for vertical upward displacement according to step D) is increased, so that the case in which the retaining device is released according to step D), which is intended for exceptional situations only, occurs less often. This ensures an overall more even milling process.

The maximum retaining force can be defined in various manners. Particularly preferably, it occurs by defining a switching threshold of a valve, in particular a pressure relief valve, especially one that operates purely mechanically. Such a valve thus switches when the counterforce acting on the retaining device in the vertical upward direction produces a pressure level in the hydraulic displacement system of the retaining device that exceeds the switching threshold of the valve, in particular of the pressure relief valve. It is also possible, however, to use an electromechanically controlled pressure relief valve.

One reason for the variation of the position of the retaining device in relation to the underlying ground is the displacement of the retaining device in relation to the machine frame of the ground milling machine, which usually occurs by means of one or more hydraulic cylinders. Moreover, the front and/or rear traveling units of generic ground milling machines are also often connected to the machine frame via lifting columns, so that a height adjustment of the lifting columns can be used, for example, to set the milling depth of the ground milling machine and to control the position of the machine frame in relation to the underlying ground. However, an adjustment of these lifting columns indirectly also causes a change in the position of the retaining device in relation to the ground. The method according to the invention thus works particularly well when the initial position of the retaining device, i.e. its vertical distance from the underlying ground, is maintained even when the lifting columns are adjusted. It is thus preferable that, when a height adjustment of lifting columns connecting traveling devices to the machine frame occurs, the height position of at least one lifting column is monitored; that a correction factor is ascertained if the elevation of the at least one lifting column is changed; and that the correction factor is used to adapt the position of the retaining device in order to maintain the defined holding distance. This correction factor is thus used to offset the change in the relative position of the machine frame, and thus the relative position of the retaining device, caused by the change in the adjustment position of the lifting columns, so that the retaining device remains in its initial position in relation to the underlying ground, i.e. maintains the defined holding distance. The height position of the at least one lifting column can be ascertained or monitored, for example, via a suitable displacement sensor which monitors and ascertains the position of two elements of the lifting column that are moved in relation to one another. This can be, for example, a cable sensor or also a capacitive displacement sensor.

Specifically, the retaining device is preferably held in the initial position in such a manner that the defined holding distance above the underlying ground is in a range from 3 to 100 mm and in particular from 5 to 50 mm. This distance refers to the vertical distance of the bottom edge of the retaining device from the surface of the underlying ground in the initial position, i.e. in a state of the ground in front of the milling drum in which it has not yet been influenced by the milling process. In this distance range, the retaining device is not only raised to a sufficient extent to avoid getting caught by smaller ground irregularities commonly encountered, but is also not yet adjusted upwards to an extent that would allow an uncontrolled ejection of significant amounts of milled material in the milling direction to the front and out of the milling drum housing.

Another aspect of the invention relates to a ground milling machine, in particular a ground milling machine designed to carry out the method according to the invention. Such a ground milling machine comprises, in a manner known per se, a machine frame supported by traveling devices, an operating platform from which the ground milling machine is operated, a milling device with a milling drum that is arranged inside a milling drum housing and that is rotatable about a rotation axis, a retaining device that is arranged in front of the milling drum in the milling direction and that is height-adjustable by means of an actuator, and a drive engine with which the drive power required for the milling and traveling operation is generated. According to the invention, the ground milling machine further comprises a positioning control unit. The positioning control unit is designed so as to adjust the actuator of the retaining device so that the actuator initially maintains the elevation of the retaining device in an initial position at a defined holding distance above the underlying ground during the milling operation, so that the retaining device is not in contact with the underlying ground. The positioning control unit is further designed in such a manner that it keeps the retaining device in the initial position with a retaining force up to a defined maximum retaining force when ground material presses against the underside of the retaining device from below with a counterforce acting at least partially in the vertical upward direction. Thus, instead of immediately yielding in the upward direction upon being contacted from below by ground material, the retaining device initially maintains its initial position. According to the invention, the positioning control unit here is designed so as to release the retaining device for a vertical, upward adjustment if the counterforce exerted on the retaining device by the ground material, e.g. clumps of material, exceeds the maximum retaining force. To this end, the positioning control unit can determine and monitor the counterforce currently acting on the retaining device either directly, for example via a force sensor, or indirectly, for example via pressure values from pressure sensors in the hydraulic adjustment system of the retaining device, and release the retaining device for upward displacement when the maximum retaining force is exceeded. Alternatively, the positioning control unit can also be designed to act purely mechanically in this situation, for example by means of a mechanically actuated pressure relief valve. It is also possible to use an electromechanically actuated pressure relief valve. How far the retaining device yields in the upward direction can also vary. The retaining device is preferably pushed upwards while maintaining the maximum retaining force, while the maximum displacement distance of the retaining device is preferably limited by means of a mechanical limiting stop. What is essential here is that, once the counterforce falls back below the maximum retaining force, the positioning control unit returns the retaining device to the initial position and no further. The retaining device is then lowered back to its initial position.

The positioning control unit is preferably designed so as to comprise a setting mode. In the setting mode, the retaining device is, usually prior to the commencement of the milling operation, raised or moved into its initial position. To this end, the positioning control unit can be designed to use an elevation of the retaining device when it is placed on the underlying ground and lifted from there to the initial position. Additionally or alternatively, the positioning control unit can also use the measurement of an operating variable associated with the height adjustment of the retaining device, for example a sliding position of an actuating cylinder of the retaining device, and, based on this, adjust the raised position of the retaining device until a target value of the operating variable associated with the height adjustment of the retaining device is reached that corresponds to a reaching of the defined holding distance. Additionally, or alternatively, it is also possible to provide a measuring device, which is used to ascertain the actual distance of the retaining device from the underlying ground, while the positioning control unit uses these values for the adjustment of the retaining device to the initial position. For example, this can occur by means of a distance sensor, e.g. a laser or ultrasound sensor, at the retaining device or a part that is moveable together with the retaining device. Additionally, or alternatively, the positioning control unit can also be designed to use a derivation of an actual raised state of the retaining device with the aid of displacement sensors associated with lifting columns for traveling devices and/or with the aid of a displacement sensor associated with a lifting device of the retaining device and/or with the aid of a position sensor of the ground milling machine. In this embodiment, the positioning control unit can thus, for example, ascertain a correction variable based on current raised positions of the lifting columns which influence and change the position of the retaining device in relation to the underlying ground.

The ground milling machine preferably comprises a display device on the operating platform of the ground milling machine which displays at least one of the following operating states. For example, the display device can be used to signal that the retaining device is in its initial position and/or is in contact with the ground, i.e. is in the zero position and/or the retaining device is pushed upwards beyond its initial position and/or has traveled the maximum upward displacement distance, for example, by hitting a mechanical limiting stop. The latter can be sensed, for example, by means of a suitable contact sensor. In particular when the retaining device is pushed upwards as far as it will go, further signals can be displayed to the operator on the operating platform, and/or an intervention in the control of the machine, for example a work stoppage, can be triggered.

Individual requirements, and in particular also the optimal initial position of the retaining device, can vary depending on the underlying ground material. Therefore, a setting device is preferably provided on the operating platform of the ground milling machine which is configured to allow at least a setting of the initial position of the retaining device and/or a setting of the maximum retaining force and/or a switching between a ground contact mode, in which the retaining device is guided so as to drag over the underlying ground in a manner known per se in the prior art, and a holding mode in accordance with the invention, in which the retaining device is held in the initial position at a distance from the underlying ground.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail in the following based on the examples shown in the figures, which show schematically:

FIG. 1: a side view of a ground milling machine of the center-rotor type;

FIG. 2: a side view of a ground milling machine of the rear-rotor type;

FIG. 3: a frontal view of the milling device against the milling direction with the retaining device in the initial position;

FIG. 4: the frontal view from FIG. 3, in which ground material contacts the retaining device in its initial position from below;

FIG. 5: the frontal view of FIGS. 3 and 4, in which a clump of ground material pushes the retaining device upwards beyond the initial position;

FIG. 6: a schematic side view of the retaining device shown in FIGS. 3 to 5;

FIG. 7: a flow chart of a method according to the invention;

FIG. 8: a control loop for the illustration of the height adjustment of the retaining device in response to the action of external forces; and

FIG. 9: a flow chart for an adaptation of the initial position when a lifting column is adjusted.

DETAILED DESCRIPTION

Identical or functionally similar components are designated with identical reference numbers in the figures, while not every recurring component is necessarily designated in every figure.

FIG. 1 shows a ground milling machine 1 of the center-rotor type with a machine frame 4 supported by traveling devices 2 via height-adjustable lifting columns 3. Arranged on the machine frame 4 is an operating platform 5, from which an operator located on the operating platform can control the traveling and/or milling operation of the ground milling machine 1. The ground milling machine 1 further comprises a milling device 6 with a milling drum housing 7 and a milling drum 8 (merely suggested in FIG. 1) arranged inside the milling drum housing, as well as a conveyor belt 9 with which milled material can be loaded onto a transport vehicle. During milling operation, the ground milling machine travels over the underlying ground in the main working direction A, which in the present case is the forward direction of the ground milling machine 1, and mills the material in the underlying ground 14 at a milling depth FT. The milling drum housing comprises a retaining device 10 (merely suggested in FIG. 1) arranged in front of the milling drum 8 in the main working direction A, lateral shields 11, as well as a rear stripping shield 12, all of which are height-adjustable in the vertical direction and at least partially close off the milling drum housing to the front, the sides and the rear, for example in order to prevent milled material from flying around and to ensure a controlled transport of the material inside the milling drum housing.

FIG. 2 shows a ground milling machine of the rear-rotor type, reference being made here to the corresponding description of the ground milling machine 1 according to FIG. 1 in this regard. In contrast to FIG. 1, only the two rear traveling devices 2 are connected to the machine frame 4 via height-adjustable lifting columns 3 in the ground milling machine 1 according to FIG. 2. At the front, the ground milling machine 1 according to FIG. 2 has either a single wheel or a pair of wheels connected with each other via a pendulum axle.

The ground milling machines 1 according to FIGS. 1 and 2 further comprise a positioning control unit 13, which controls and regulates the height adjustment of the retaining device 12, as explained in greater detail below.

FIGS. 3, 4 and 5 are front views of the milling drum 6 against the main working direction A. In FIG. 3, the retaining device 10, which protrudes from the rest of the milling drum housing 7 in the main working direction A and which is connected here to a front delimiting wall 15 and is height-adjustable together with the same in the vertical direction via two hydraulic cylinders 16, is in its initial position. In this initial position, the retaining device is, with respect to its bottom edge 17, which in the present case is formed by runners, at a distance A in the vertical downward direction from the surface of the underlying ground 14. The retaining device 10 is thus in a slightly raised position and is thus not in contact with the underlying ground 14. In the present example, the specific distance A is 10 mm and can usually lie in the value range indicated above. As is evident from FIG. 3, the milling tools, which are arranged on the milling drum 8 behind the retaining device in the working direction A, engage the underlying ground. The raised initial position of the retaining device 10 is maintained by the two hydraulic cylinders 16, the extended position of which is controlled by the positioning control unit 13, as suggested by the dotted lines in FIGS. 3, 4 and 5.

During milling operations, it can occur that, due to the milling drum rotating against the direction of rotation of the traveling devices, ground material, in particular in the form of ground clumps 18, is lifted in the vertical upward direction in relation to the remaining ground surface and strikes the bottom edge 17 of the retaining device 10 in its initial position from below. This state is shown in FIG. 4. In this case, the positioning control unit 13 can now be designed so as to apply a retaining force which counteracts this vertically upward acting counterforce exerted by the ground material, i.e. presses downwards in the vertical direction, and thus initially keeps the retaining device 10 in the initial position.

However, if the counterforce produced by the ground material rising in the vertical upward direction exceeds a defined maximum retaining force, provision is made for the retaining device 10 to yield in the vertical upward direction, as shown in FIG. 5. Here, the retaining device is raised in relation to the remaining ground surface of the underlying ground 14 by the additional amount A+ relative to the initial position. In the present example, the extent to which the retaining device 10 can be pushed further upwards beyond the initial position is limited by a mechanical limiting stop. Provision is also made for the positioning control unit 13 to continue to apply the maximum retaining force to the retaining device 10 in the vertical downward direction by means of the hydraulic cylinders 16.

If the counterforce applied in the vertical upward direction by the ground material now falls below the maximum retaining force, the retaining device 10, controlled by the positioning control unit 13, returns to its initial position and reassumes its initial position shown in FIGS. 3 and 4. Thus, the retaining device 10 is lowered in the vertical downward direction only as far as the initial raised position in relation to the ground surface, and not down to the ground surface.

FIG. 6 illustrates, as an example, further details of the positioning control unit 13, as well as some essential elements of the ground milling machine 1. In the present example, it comprises several sensors 19, 20 and 21, the measuring signals of which are transmitted to a processing unit 23 with suitable control software. The sensor 19 is a displacement sensor integrated into the lifting column 3. Such a displacement sensor can be integrated in each of the provided lifting columns of the ground milling machine 1. With the aid of these sensor data, the positioning control unit 13 can thus ascertain the position of the machine frame and thus indirectly the position of the retaining device (at least partially). The sensor 20 is a distance sensor which is height-adjustable together with the retaining device 10. The distance sensor 20 thus ascertains the vertical distance of the retaining device 10 from the underlying ground. The sensor 21 is a displacement sensor inside the hydraulic cylinder 16 by means of which the retaining device 10 is height-adjustable in the vertical direction. In particular with the aid of this sensor, the positioning control unit 13 can thus track how far the retaining device 10 has been pushed upwards or downwards starting from a known position. The processing unit 23 of the positioning control unit 13 controls a hydraulic system supply unit 24 comprising, for example, a pump, a pilot valve or the like. With the aid of the hydraulic system supply unit 24, it is possible, for example, to produce the additional retaining force beyond the force of gravity. Alternatively, it is also possible to produce the retaining force by switching a hydraulic valve to a blocking position when a counterforce increases. Also provided is a pressure relief valve 25, which can be controlled, for example, by the processing unit 23 of the positioning control unit 13 or can also be designed to act purely mechanically. Via the pressure relief valve 25, it is possible, for example, to define the maximum retaining force specifically by means of the switching threshold of the pressure relief valve. FIG. 6 is to be understood merely as an example. Additionally or alternatively, pressure sensors or similar sensing means can, for example, also be provided inside the hydraulic system supply unit of the hydraulic cylinders 16 and the lifting columns 3.

FIG. 7 illustrates the essential method steps for controlling the position of the retaining device in accordance with the invention. A first essential aspect is that, in step 26, the retaining device 10 is held in the initial position during the milling operation, for example as described above. In the initial position, the retaining device is raised in relation to the underlying ground and is thus not in contact with the latter. Thus, the retaining device 10 does not drag over the ground when the ground milling machine 1 travels over the underlying ground in the working direction A during the milling operation. Step 27 involves starting the milling operation, which in the present case also includes a continuation of a milling operation. Here, the retaining device is still held in its raised initial position according to step 26, in particular in relation to the machine frame. It can now occur that the motion of the milling drum causes ground material to rise from below and press upwards against the retaining device with a counterforce. In the method shown in FIG. 7, step 28 then involves initially maintaining the retaining device in its initial position and applying thereon a retaining force, which is produced, e.g., by the weight of the retaining device or also by additional application of pressure by means of the hydraulic cylinder 16, and which counteracts this counterforce. However, if the counterforce exerted on the retaining device by the ground material in the vertical upward direction continues to increase and a maximum retaining force is exceeded, step 29 involves releasing the retaining device so that it now yields by a displacement upwards. This can include, for example, switching a pressure relief valve. This yielding motion is usually limited in the vertical upward direction, for example by means of a mechanical limiting stop. If the counterforce exerted by the ground material pressing against the retaining device in the vertical upward direction from below now decreases enough to fall back below the maximum retaining force, a returning of the retaining device towards the initial position finally occurs in step 30. This returning movement is limited in the vertical downward direction by the reaching of the initial position of the retaining device. The retaining device is thus not pressed down to the normal ground surface but only as far as the initial position, in which the retaining device is positioned at the defined holding distance A above the underlying ground.

Prior to the method steps described above, step 31 can also involve moving the retaining device to the initial position by means of further measures of the method. For this purpose, it is, for example, possible that, in order to ascertain the zero position, i.e. the position in which the retaining device contacts the ground surface, the retaining device is in step 31 initially placed on the underlying ground, which can be ascertained, for example, by a pressure fluctuation in the hydraulic system supply unit of the hydraulic cylinder 16, and is then raised by a defined distance, which can be measured, for example, with the aid of the sensor 21, until it reaches the initial position. Additionally or alternatively, it is also possible that the vertical distance of the retaining device from the underlying ground is measured and monitored by the distance sensor 20 and is ultimately used as a target variable for the height adjustment of the retaining device.

Each of the method steps described above can comprise the use of a correction variable, in particular for defining and monitoring the initial position of the retaining device, in the control and regulation of the position of the retaining device according to step 32. This can become relevant in particular when the ground milling machine 1 has a machine frame that is height-adjustable via lifting columns, since the adjustment of the lifting columns also results in a change in the position of the retaining device in relation to the underlying ground. In this case, provision can be made for the positioning control unit 13 to receive displacement data obtained via suitable displacement sensors of the lifting columns and to consider these data for the control and regulation of the position of the retaining device. Additionally or alternatively, it is also possible here to use a position sensor of the ground milling machine, with which in particular the horizontal position of the machine frame is monitored. Specifically, this can mean that, when the machine frame is lowered by retracting the lifting columns, which also results in a lowering of the retaining device indirectly connected to the machine frame, the retaining device is raised by the amount of the correction factor in order to remain in its initial position in relation to the ground surface, i.e. in order to still have the same vertical distance from the ground surface after the height adjustment of the machine frame.

FIG. 8 now in particular illustrates further details of an illustrative design of a control loop, in particular for the method steps 28 to 30. The starting point according to 33 is the currently set height of the retaining device in the vertical direction above the surface of the underlying ground. Starting from the current position, which is ascertained, for example, with the aid of one of the sensors 20 or 21, the positioning control unit 13 conducts a check 34 whether the current position of the retaining device deviates from the initial position A in the vertical upward direction (h>0). If there is such a deviation, the retaining device remains in this floating position, if necessary with an additional retaining force being applied. If, on the other hand, there is no such deviation, according to 36, the retaining device remains held or fixed in the initial position. At 37, a determined position of the retaining device is updated. The current position thus determined is fed back and the control loop described above is run again. The process described in FIG. 8 can be performed by the positioning control unit 13 continuously or also intermittently, for example in a timed manner.

FIG. 9 illustrates further details for the adaptation of the positioning of the retaining device or the determination of a correction variable for the adjustment of the retaining device in order to maintain the initial position, i.e. the vertical distance of the underside of the retaining device from the underlying ground, when the lifting columns 3, in particular in a ground milling machine 1 as shown in FIG. 1, are adjusted. The starting point here is a measurement 38 of the position of the lifting columns 3, in particular at least the lifting columns 3 connecting the traveling devices to the machine frame at the front right and the front left, respectively. This measurement 38 can be performed during the milling operation or also at the beginning of the milling operation. If, in step 39, the positioning control unit 13 determines a change in the lifting height of the right and/or front left lifting columns, the calculation of a correction factor or correction variable occurs in step 40 for the adjustment of the retaining device based on the new height information. For example, if the front lifting columns have been retracted, the correction factor is the correction variable by which the retaining device needs to be raised in order to reassume its initial position, i.e. the defined holding distance of the retaining device in relation to the ground surface in the vertical downward direction. In step 41, the retaining device is now raised or lowered based on the determined correction variable. If, on the other hand, no change in height is determined, the retaining device remains fixed in its current raised position in step 42. Subsequently, the control loop described in FIG. 9 is run again starting at step 38. It is possible here to run the process described in FIG. 9 continuously. However, the adaptive height adjustment of the retaining device due to a height adjustment of lifting columns as shown in FIG. 9 preferably occurs only if the lifting columns are actuated. This method also applies accordingly when the height of the rear lifting columns and/or of the front and rear lifting columns of a ground milling machine is adjusted. 

What is claimed is:
 1. A method for controlling the elevation of a retaining device of a ground milling machine, comprising the steps: a) holding the retaining device in an initial position at a defined holding distance above the underlying ground, so that the retaining device is not in contact with the underlying ground; b) starting the milling operation; c) if ground material presses against the underside of the retaining device from below with a vertical upward counterforce, maintaining the retaining device in the initial position with a retaining force up to a defined maximum retaining force; d) if the counterforce exceeds the maximum retaining force, releasing the retaining device for vertical upward displacement and a pushing of the retaining device upwards by the counterforce; and e) returning the retaining device towards the initial position when the counterforce falls below the maximum retaining force, wherein the retaining device is returned no farther than to the initial position of the retaining device.
 2. The method according to claim 1, wherein at least one of the following steps precedes step a): placing the retaining device on the underlying ground and subsequently raising the retaining device until the initial position at the defined holding distance of the retaining device from the underlying ground is reached; measuring an operating variable associated with the height adjustment of the retaining device and setting the raised height of the retaining device until a target value of the operating variable associated with the height adjustment of the retaining device is reached that corresponds to a reaching of the defined holding distance of the initial position; measuring a distance variable that changes with the height adjustment of the retaining device in relation to the underlying ground and raising or lowering the retaining device until a distance variable is reached that corresponds to the defined holding distance of the initial position; and deriving an actual raised state of the retaining device with the aid of displacement sensors associated with lifting columns for traveling devices and/or with the aid of a displacement sensor associated with a lifting device of the retaining device and/or with the aid of a position sensor of the ground milling machine, and raising or lowering the retaining device until a target raised state of the retaining device is reached that corresponds to the defined holding distance of the initial position and/or until a target displacement value of the displacement sensor associated with the lifting device of the retaining device is reached that corresponds to the defined holding distance of the initial position while considering measurement data of the position sensor of the ground milling machine.
 3. The method according to claim 1, wherein a pressure force acting in the vertical downward direction is actively applied in step c) in addition to the weight of the retaining device in order to hold the retaining device in the initial position at a defined holding distance above the underlying ground.
 4. The method according to claim 1, wherein step d) comprises switching a pressure relief valve.
 5. The method according to claim 1, wherein when there is a height adjustment of lifting columns connecting traveling devices to the machine frame, the elevation of at least one lifting column is monitored, in that a correction factor is determined if the elevation of the at least one lifting column is changed, and in that the correction factor is used to adapt the position of the retaining device in order to maintain the defined holding distance.
 6. The method according to claim 1, wherein the retaining device is held in the initial position so that the defined holding distance above the underlying ground lies in a range of from 5 to 50 mm.
 7. A ground milling machine for carrying out the method according to claim 1, comprising: a machine frame supported by traveling devices; an operating platform, from which the ground milling machine is operated; a milling device with a milling drum which is arranged inside a milling drum housing and which is rotatable about a rotation axis; a retaining device which is arranged in front of the milling drum in the milling direction and which is height-adjustable via an actuator; a drive engine, with which the drive power required for the milling and traveling operation is generated; wherein a positioning control unit is provided which regulates the actuator of the retaining device in such a manner: that it initially maintains the elevation of the retaining device in an initial position at a defined holding distance above the underlying ground during milling operation, so that the retaining device is not in contact with the underlying ground; that it keeps the retaining device in the initial position with a retaining force up to a defined maximum retaining force when ground material presses against the underside of the retaining device with a vertical upward counterforce from below; that it releases the retaining device for vertical upward displacement when the counterforce exerted on the retaining device by the ground material exceeds the maximum retaining force; and that it returns the retaining device towards, but no farther than, the initial position when the counterforce falls below the maximum retaining force.
 8. The ground milling machine according to claim 7, wherein the positioning control unit comprises a setting mode so as to comprise at least one of the following features for raising the retaining device to the initial position: it uses an elevation of the retaining device when the retaining device is lowered to the underlying ground; and/or it incorporates measuring an operating variable associated with the height adjustment of the retaining device and setting the raised height of the retaining device until a target value of the operating variable associated with the height adjustment of the retaining device is reached that corresponds to the reaching of the defined holding distance; and/or it incorporates measuring a distance variable that changes with the height adjustment of the retaining device in relation to the underlying ground and raising or lowering the retaining device until a distance variable is reached that corresponds to the defined holding distance; it incorporates deriving an actual raised state of the retaining device with the aid of displacement sensors associated with lifting columns for traveling devices and/or with the aid of a displacement sensor associated with a lifting device of the retaining device and/or with the help of a position sensor of the ground milling machine.
 9. The ground milling machine according to any one of claim 7, wherein it has a display device on the operating platform of the ground milling machine, which displays at least one of the following operating states: the retaining device is in its initial position; the retaining device is in contact with the ground; the retaining device is pushed upward beyond its initial position.
 10. The ground milling machine according to claim 7, wherein it has a setting device on the operating platform of the ground milling machine, with which at least one of the following settings can be performed: setting the initial position of the retaining device; setting the maximum retaining force; switching between a ground contact mode and an initial position holding mode. 